Without photosynthesis, planet earth would look a lot more like Mars and life as we know it would not be possible.

Photosynthesis is a chemical process that takes place in many forms of bacteria and virtually all plants, including aquatic plants and algae. Using just three simple ingredients - carbon dioxide, water, and sunlight - plants and bacteria are able to make their own food.

Fortunately for all animals, including humans and fish, oxygen is a by-product of this miraculous process. As long as photosynthesis is occurring, oxygen is continuously being released into the air and into the world's lakes, oceans, rivers, and ponds.

Early forms of algae and bacteria were the first organisms to photosynthesize, more than three BILLION years ago. After a while, significant amounts of oxygen had accumulated in the atmosphere. Scientists estimate that it takes about 2,000 years of photosynthetic activity to "turn over" or replenish all the oxygen in earth's biosphere.

The ultimate lesson: photosynthesis in algae, bacteria and plants is how we're able to exist!

Basic Mechanics of Photosynthesis

In most plants, photosynthesis takes place in special cells known as chloroplasts. The green hue that we see when we look at plants is the result of tiny grains of green pigment - light absorbing molecules - found inside the chloroplasts. These pigments are commonly referred to as chlorophyll (chloro=green; phyll=leaf).

Different types of plants use different forms of chlorophyll for photosynthesis.

Chlorophyll a is the pigment directly responsible for transforming light energy (sunlight) into chemical energy (carbohydrates).

Many plants also contain chlorophyll b and chlorophyll c, pigments which help carry out other chemical processes.

A Small History Lesson Jan Ingenhousz, an Austrian physician from the 18th century, is credited with being the first one to provide visible proof that photosynthesis produces a gas. He did this by placing willow tree branches underwater in full sunlight and observing that bubbles were forming on the submersed leaves. Soon after, he figured out that leaves and stems were the only part of the plant that could "photosynthesize"; fruits use up oxygen as they ripen, but they can't produce it.

Aquatic Plants and Photosynthesis

Plants, including aquatic plants, produce oxygen, and they also use oxygen. Here's how these "invisible processes" work:

During a sunny day, dissolved oxygen is generally plentiful because photosynthesizing algae and plants are constantly releasing it into the water. Even though a myriad of organisms are using the oxygen, there is an oxygen surplus thanks to the plants.

After sundown however, things get tricky; without sunlight, photosynthesis slows down considerably or even stops. So now, in addition to the usual oxygen demands (from fish, macro-invertebrates, tadpoles, etc.), algae and plants are also pulling oxygen from the water. If there is an abundance of plants and/or animals, the system can become stressed fairly quickly and the potential for a fish kill increases, especially following several days of cloudy weather or low-light.

Factors that Influence the Rate of Photosynthesis
and Oxygen Production in Aquatic Plants

Predicting Submersed Plant Growth With a Secchi Disk Most people think of a Secchi disk as a tool for measuring water clarity. And it certainly is. However, it can also be used to predict both the abundance and type of submersed plants we would expect to find growing in a specific waterbody. Light Attenuation and Photosynthesis Plants use the same visible light spectrum that we do (wavelengths measuring between 400 and 700 nanometers). However, submersed aquatic plants have a harder time getting the light they need for photosynthesis; suspended particles, dissolved substances and water depth restrict the amount of light that penetrates the water. There are at least two ways to estimate light reduction in water depth (aka light attenuation): Secchi disk - The easiest and cheapest method to measure light reduction involves the use of a Secchi disk. The disk is lowered into the water and where it becomes invisible is the "Secchi depth". Once this measurement is obtained, you can multiply it by two to obtain a light attenuation estimate. For example, if a Secchi depth reading for your lake is five feet, then multiply it by 2 to get a light attenuation estimate of 10 feet. 2 x 5-foot Secchi depth = 10 feet of light attenuation This means that if your lake is 8 feet deep, then light is probably reaching the bottom of the lake and submersed plants can be expected to grow on the bottom. However, if the Secchi disk reading is only 3 feet, then you can predict that light is not penetrating to the bottom of an 8-foot deep lake (2 x 3-foot Secchi = 6 feet of light attenuation). In this case, growth of submersed plants attached to the bottom probably is not going to happen. Electronic light meter - The second method for measuring light attenuation involves the use an electronic light meter. The process is: a reading is taken immediately below the surface and also at different depths; then the readings are used in the following formula to calculate light attenuation: Iz = I0 e-kz Where: Iz is the intensity of light at depth I0 is the intensity of light immediately below the surface of the water. E is the natural logarithm. K is the vertical attenuation (reduction) co-efficient for the downward penetration of light (also known as "irradiance").

Water Color

Dissolved substances in the water, such as naturally-occurring tea-colored tannins, can prevent sunlight from penetrating very far down into the water column. This is one reason why many red-water or black-water lakes have few submersed plants. There simply isn't enough light to available to allow photosynthesis to occur below a certain depth. Of course, they can support plenty of emersed or floating leaved plants.

Go to another page on this web site for more about water color.

Murky (Turbid) Water

An abundance of suspended clay, silt or phytoplankton (free-floating algae) can also make a lake turbid (murky), which may slow or prevent submersed plant photosynthesis and growth due to low light conditions.

Florida's infamous Lake Apopka is a good example of the relentless cycle that can occur in a turbid lake: due to an abundance of algae and suspended sediments in the water, submersed plant communities have not been able to re-establish themselves on the bottom. Without sufficient soil-anchoring plants on the bottom, sediments continue to be re-suspended in the water column, furthering the turbidity problem.

Cloudy Weather

Several days of cloudy weather can slow the rate of photosynthesis, resulting in lower oxygen production within a waterbody. If you have a small fish pond, during cloudy weather is a good time to turn on the aerator. Click here to view a pdf about The Role of Aeration in Pond Management.

Day Length

Fewer daylight hours during autumn and winter also influences photosynthesis, as well as temperature and humidity. (The same can be said for terrestrial plants.)

Leaf Characteristics

Changes in a leaf's condition (aging, tearing, etc.), the arrangement of leaves on a branch, even the shape and size of a leaf affects the amount of photosynthesis that occurs within a plant; so much so that two different types of plants have developed over the millennia, based on their tolerance for light.
"Sun plants" experience an increased rate in photosynthesis as light intensity increases. Leaves on a "sun plant" tend to be smaller and thicker with more pronounced lobes than shade leaves. Special cells within these leaves allow for higher rates of photosynthesis.

"Shade plants" photosynthesize at a lower rate, even if there is a lot of light available. Their leaves tend to be thinner and longer, with fewer chlorophyll cells - making it easier to photosynthesize under low light conditions.

While photosynthesis is similar in both aquatic and terrestrial plants, there are a few important differences. For example, emersed plants, floating-leaved plants and terrestrial plants extract needed carbon dioxide from the air, while submersed plants have to get it from the water.

Also, submersed plants have a harder time obtaining carbon dioxide (C0₂) from the aquatic environment due to a lower exchange rate of gases. (The diffusion of gases in water is 104 times slower than in air.) To compensate for this, some plants have developed leaves that grow above the water, allowing them to pull C0₂ from the air as well.

Some aquatic plants like hydrilla verticillata have developed the ability to live in low light conditions. This means it can grow in deeper water and colonize in a lake much earlier in the growing season than other plants. It can also become established in lakes with high turbidity when others cannot - which constitutes quite an advantage. No wonder it has become the most invasive plant in Florida.